Generally, systems for analyzing a sample in a flow cell are pressure driven fluidic systems using pressure pumps. Pressure driven fluidics systems have several disadvantages. One disadvantage is that pressure driven systems require the sample vessel to be sealably engaged to the flow cell assembly. This makes removal of the flow cell more complicated, because removal of the flow cell can produce hazardous aerosols. Pressure systems are also known to develop system leaks due to the pressure and may require frequent replacement of lines and valves. Additionally, pressure driven systems can introduce contaminants into the sample. Another disadvantage of pushing fluid through the system is that air can become trapped in the system or air bubbles can be introduced into the sample. Introduction of air into the pump can cause cavitation resulting in shock to the system. Moreover, in pressure driven systems, it is difficult to adequately purge the lines after each sample has been tested. This can result in residual material being left in the system when the next test is performed. Also, purging the system using air pressure tends to cause bubbling or foaming in the samples, which may introduce inaccuracies to the analysis.
The prior art vacuum driven systems that have been used to analyze samples in a flow cell also have disadvantages. In these prior art systems, a vacuum pump is directly connected to the flow cell. Again, the use of a pump can cause air bubbles to be introduced into the sample and air trapped in the pump transmit shock to the system. Additionally, the continuous on and off cycle of the pump can result in uneven passage of a sample through the flow cell. Prior art vacuum systems are also generally suited for passing multi-cell samples through the flow cell. Having a pump directly connected to the flow cell can negatively impact single-cell samples, in part, because of the shock transmitted to the system.
In analyzing microfluidic volumes and related biological materials using a light source, it is desirable for the light source to hit the sample in such a way that results in total internal reflection fluorescence (“TIRF”). TIRF is an optical phenomenon that occurs when light propagating in a dense medium, such as glass, meets an interface with a less dense medium such as water. If the light meets the surface at a small angle, some of the light passes through the interface (is refracted) and some is reflected back into the dense medium. At a certain angle, known as the critical angle, all of the light is refracted. However, some of the energy of the beam still propogates a short distance into the less dense medium, generating an evanescent wave. The evanescent wave only penetrates about 100 nm into the medium. If this energy is not absorbed, it passes back into the dense medium. However, if a flourophore molecule is within the evanescent wave, it can absorb photons and be excited. The excited fluorophores can be observed using, for example, an intensified CCD camera. Accurately maintaining the critical angle to obtain TIRF in a dynamic system is difficult.